Children and women said to be among victims of attacks on two towns in Deraa apparently prompted by army defections.

Six children are among at least 57 people killed in southern Syria after the army launched an all-out assault on two towns in Deraa province, according to a London-based activist group.

The Syrian Observatory for Human Rights (SOHR) identified the towns on Thursday as Sanamin and Ghabagheb.

"At least six children, seven women, 16 rebel fighters, 16 other unidentified men and 12 army troops were killed on Wednesday, in fighting, shelling and summary executions waged after the army launched an assault on Sanamin and Ghabagheb," the SOHR said.

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Al Jazeera's Bernard Smith, reporting from Istanbul in neighbouring Turkey, said the raid in which 45 people reportedly died in Sanamin, about 50km south of Damascus, was precipitated by a rash of defections by members of the Syrian army.

"So, a day later, the Syrian army had gone into this area to try and find these soldiers, perhaps try and kill them to stop them from giving intelligence to the opposition but also as a warning, to ... [other] soldiers, that if they consider defection, then this is what's going to happen to them," he said.

"But of course, as we see on daily basis, caught up in these battles are civilians as well."

Lebanese town hit

In another development on Thursday, Syria's air force attacked a rural area near the town of Arsal in eastern Lebanon for the second time in 24 hours, Lebanese officials said.

"I can confirm there was a raid," Ahmad Fliti, deputy head of the council in the majority Sunni Muslim town in northeastern Lebanon, told the AFP.

"Several ambulances travelled immediately to the affected area to transfer the wounded to clinics and hospitals."

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Fliti said was unclear how many people were hurt.

A security official confirmed reports of the attack, and a Lebanese Red Cross said the organisation took four wounded away from the scene.

The raid came a day after Syrian jets bombed Sarjal Ajram, the same area that was struck on Thursday.

The cross-border raids occurred as a prominent rights monitor said Syrian military air strikes have hit bakeries and hospitals among other civilian targets and killed thousands of people in raids that it said amount to war crimes.

"Individuals who commit serious violations of the laws of war wilfully, that is intentionally or recklessly, are responsible for war crimes," Human Rights Watch (HRW) said on Thursday in a report titled Death from the Skies .

Bakeries targeted

Basing its findings on investigations in rebel-held areas of three war-torn provinces, the New York-based HRW documented air strikes on four bakeries and two hospitals, along with other civilian targets.

One hospital in the northern city of Aleppo, the Dar al-Shifa, suffered at least four attacks alone.

"In village after village, we found a civilian population terrified by their country's own air force," Ole Solvang, a Human Rights Watch emergencies researcher, said.

"These illegal air strikes killed and injured many civilians and sowed a path of destruction, fear, and displacement/"

Citing a network of activists, HRW said that "air strikes have killed more than 4,300 civilians across Syria since July 2012."

The report detailed the use of highly explosive munitions that sometimes flattened several houses in a single attack.

Fragmentation bomb

One resident of the northern town of Azaz told HRW that at least 12 members of his family were killed in a bombing of their homes on August 15 last year.

One of the explosive devices used in attacks on Azaz was a powerful fragmentation bomb "that has a casualty-producing radius of 155 metres", HRW said.

Other types of munitions used by the Syrian army were cluster bombs, ballistic missiles and incendiary weapons, HRW said.

Syria responded to the HRW report by accusing foreign organisations of being biased in favour of opposition fighters.

Without naming the group, Syrian state television said on Thursday that foreign organisations rely only on accounts by opposition activists when reporting about the conflict.